1975: Capacitive touch switches in use

1. 1975: Capacitive touch switches in use

2. 1994: 1st laptop capacitive touchpad

3. 2003: 1st iPod capacitive touchwheel

4. Today: Multi-touch capacitive touch screens How do they work?

5. CNMAT Sensor Workshop 2008 Capacitive Touch Sensors 2008-7-23 John Lazzaro CS Division, UC Berkeley www.cs.berkeley.edu/~lazzaro

6. Today’s lecture: Capacitive touch sensing Physics of capacitance Simple touch switches Touch pads and touch screens Novel applications

7. Physics of Capacitance

8. Capacitor: A part you buy from Digikey.

9. Capacitor: Two conductive plates, separated by an insulator (dielectric). Current cannot flow through an insulator. Thus, electrons can’t pass from A to B.

10. Battery pumps electrons from plate A to plate B. We notate each electron pumped from A as and refer to it as a positive charge.

11. How many It depends on the 3-D shape of the capacitor, and the material properties of the dielectric. A: Area of plates d: Plate separation ϵ: Dielectric property Q: Number of V: Voltage on plates does a 1.5V battery place on plate A? The ratio Q/V is defined as the capacitance C of the device. Q = CV

12. We can use a capacitance meter to measure C. Conceptually, the meter puts a V across the C, and then drains off the charge and counts Q. C = Q/V. LED Display

13. Notation alert! From now on we draw net charge on the plates. field lines We draw field lines to illustrate the pairing of charges. A capacitance meter counts the number of field lines (and thus, Q).

14. (Sources: Analog Devices, Cypress, and Synaptics data sheets and websites). Simple Touch Switches

15. What’s on the back of the PC board? Capacitance meter chip (Synaptics) Traces on back of PC board form the “capacitors”! iPod 3G front panel

16. PC trace design for a capacitive button switch PC Board Top View Capacitor Plate A FR-4 (insulator) iPod White Plastic Casing PC BOARD Cross- Section FR-4 Copper Copper Copper field lines field lines Capacitor Plate B Copper Plate A Plate B What does the finger do? Glass

17. Some of the field lines will terminate on the iron in the red blood cells of a nearby finger. Capacitor Plate A Capacitor Plate B Recall Capacitance meter counts the number of field lines to determine Q, and then computes C = Q/V

18. Compact button switch designs LED LED Capacitively sensed mechanical switch Capacitor Plate A Capacitor Plate A Capacitor Plate B Capacitor Plate A Capacitor Plate A Capacitor Plate B Capacitor Plate B

19. Capacitive fader design Capacitor Plate B0 Plate B1 Plate B2 Plate B3 Capacitor Plate A Plate B4 Plate B5 Plate B6 Plate B7 Super-resolution via interpolation ...

20. Touch wheel design B0 B1 B7 B2 Plate A B6 B3 B5 B4 B1 B0 B2 A B7 B3 B6 B4 B5

21. Keypad design Switch array One switch B0 B1 A B0 B1 A B0 B1 (connect all As together) If row I and column J have big Cs, key Kij is touched

22. Scaling up

23. Typical part $3.04 Qu 1 $1.69 Qu 5000 Senses 14 Cs

24. Sense time per pad: 3ms. If all 14 channels in use: 36ms “frame rate”. Measured pad C is always drifting. Sensor chip tracks it adaptively. Plastic thickness over PCB: 2-4mm. Pad C a few pF. Finger ΔC a few fF ! Interesting AD7142 facts ...

25. Touchpads and Touchscreens (Sources: Apple, Synaptics, and Cirque patents, various websites).

26. Touchpad: A circuit board Top view - finger surface Back view: Capacitance meter chip

27. Touchpad Cross-section Grid cross-points are sensor capacitors.

28. Touch screens use a transparent wire matrix (ITO, Indium-Tin Oxide) under glass or plastic.

29. The Sensor Array Recall ... Cross-Point close-up Capacitor Plate A Capacitor Plate B

30. Some of the field lines will terminate on the iron in the red blood cellsof a nearby finger. Sensor Array If many fingers may touch at once (multi-touch), scan out Cij array (frames) at a constant frame rate. “Capacitance video camera”.

31. Like a camera chip readout Excite one row with a sense waveform Read capacitance of all columns in parallel ... Circuit from Apple patent teaching the iPhone touchscreen controller chip. To parallel A/D channels

32. Not feasible in 1994 ... Looks a lot like a radio receiver ... Which may be why Apple partnered with Broadcom for the design! One A/D readout channel

33. Before Multi-touch Single-touch: A fast accurate keypad Keypad recap: Switch Array adapted the keypad idea to track one finger over a track pad cross-point matrix (1991). B0 B1 Connect all As together If row I and column J have big Cs, key Kij is touched One switch

34. Before Multi-touch First, drive all Y lines simultaneously, and measure X capacitance vector ... Then, drive all X lines simultaneously, and measure Y capacitance vector. B0 B1 Track peaks over time, etc ...

35. All-analog computation ! A/D at very end. Analog peak interpolation Analog background C normalization To identical circuits for Y.

36. For your design: A typical part Recently, standard parts have started to appear ... Most 2-D products are sold as per-customer custom modules (Synaptics) or done as in-house ASICs (Apple).

37. Novel applications

38. Touch sensing ideas ... Techniques not limited to finger capacitance

39. Touch pad and touch screen ideas ... Techniques are not limited to the Cartesian coordinate space.

40. Touch sensing ideas ... Techniques can be adapted to flexible printed circuit technologies.

41. Touch user interfaces on curved surfaces (Apple patent)

42. Touch user interfaces on curved surfaces (Apple patent)

43. Final thought Touch Sensing = Materials + Electronics + Product Design A decade ago, a design team needed experts in all 3 disciplines to succeed. Today, sensor chips and Internet PC board services change the equation. Sensing on curved surfaces await their iPod moment -- a product design concept that brings them mainstream.